Selector switch



Dec. 18, 1934. R. RUMMELT y 1,984,703

SELECTOR SWITCH Filed Dec. 9, 1932 Patented Dec. 18, 1934- PATENT OFFICE1,984,703 SELECTOR swrrorr Reuben Rummelt, Durham, N. C., assignor toAssociated Electric Laboratories, Inc., Chicago, Ill., a corporation ofDelaware Application December 9, 1932, Serial No. 646,418

7 Claims.

This invention relates in general to selector switches, andparticularlyto selector switches of the Strowger step by step type such as are usedin automatic telephone systems. These systems as widely used in the pasthave been arranged so that the trunk lines to which the selectorswitches have-"access are made busy by grounding the test/conductors,and the selectors in hunting for idle trunks lsearch for absence ofground. 'Ihe object of the invention is an improved circuit arrangementfor selector switches of this type which will largely prevent twoselectors from seizing the same idle trunk in the event that they startto hunt simultaneously or substantially so. The improvement consists inadding to each selector switch a resistance which is connected betweenthe incoming test conductor and the test wiper. The addition of theseresistances greatly modies the action of the selectors in testing foridle trunks when two or more of them are testing at the same time. Theeffect is to substantially eliminate double seizure.

Further details of the operation of the improved selector circuit willbe pointed out fully hereinafter, reference being made to theaccompanying drawing.

Referring to the drawing, the reference character S indicates a Strowgerstep by step selector switch having the improved circuit referred to.The mechanism is understood to be of the usual type, and. comprises avertical and rotary shaft controlled by the vertical magnet 9 and therotary magnet 10. The shaft carries three wipers indicated by thereference numerals 20, 21 and 22. A release magnet 8 is provided forrestoring the switch to normal. The switch also has the usual relays forcontrolling its operation, as will be subsequently described. Theresistance which is added in accordance with this invention is indicatedby the reference numeral 30.

The selector S' is in all respects like the selector S. It is shown forconvenience in describing the operation and effect of the new improvedcircuit arrangement, 4which as before stated, deals with the preventionof double seizure by two selectors which start to hunt simultaneously.

At the right of the drawing a trunk line comprising conductors 26, 27and 28 is indicated.

This trunk line is accessible to the selector S as shown and is alsoaccessible by the usual trunk multiple to the selector S' and to otherselector switches.

The normal operation of the selector S will rst be briey described. Whena calling subscriber seizes the selector S a circuit is completed overthe incoming line conductors 2 and 4 for the line relay A. Uponenergizing, relay A completes a circuit for the slow acting releaserelay B at armature 5. Relay B operates and at armature 7 it prepares acircuit for the vertical magnet 9. At armature 6 relay B connectsground` to the incoming release or test conductor 3, thereby making theselector S busy and completing a holding circuit for the precedingswitch or switches.

When the calling subscriber dials the digit which is to operate theselector S, one or more interruptions are produced in the circuit ofline relay A in the well known manner. Assuming that the digit l iscalled, there will be one interruption, and the line relay A willmomentarily retract its armature 5 once. An impulse is therebytransmitted to the vertical magnet 9 through the slow-acting seriesrelay C. Magnet 9 steps the switch shaft up so that the wipers 20, 21and 22 stand opposite the rst level, in which it may be assumed that thetrunk line comprising the conductors 26, 27 and 28 terminates. Relay Cenergizes in series with the Vertical magnet and is designed to `retainits armature retracted throughout a series of impulses. Upon energiz-,ing, relay C prepares a circuit for the stepping relay E at armaturel2, which circuit is com! pleted upon the closure of the shaft springs19 when the shaft begins to rise. Relay E energizes and at armature 14prepares a circuit for the rotary magnet 10. At armature 13-relay Ecompletes a locking circuit for itself. When relay C falls back upon thecessation of impulses to the vertical magnet 9, it completes a circuitat armature l2 for the rotary magnet 10, and also breaks the initialenergizing circuit for the stepping relay E. This relay is, however,locked energized for the time being. The rotary magnetlo now energizesand rotates theshaft so that the wipers 20, 2l and 22 rotate in on thefirst level of bank contacts, the said wipers making contact `with bankcontacts 23, 24 and 25, respectively. The rotary magnet 10 also opensits interrupter contacts and thereby breaks the circuit of the steppingrelay E.' The relay E therefore falls back and opens the circuit of therotary magnet and also its own locking circuit.`

The switch wipers are now standing on the contacts assigned to the firsttrunk of the level, and the further operation depends on whether or notthis trunk line is busy. If the trunk' line is busy, there will be aground potential on test contact 24, and a new energizing circuit willbe completed for the stepping relay E which includes the test wiper 21.This circuit is also effective to prevent the energization of theswitching relay D. Assuming therefore that the rst trunk line is busy,the stepping relay E is again energized, and the same series ofoperations that have just been described occur again, resulting in therotation of the switch wipers to the set of contacts assigned to thesecond trunk line. These operations continue to be repeated until thetest wiper 21 arrives at an ungrounded test contact similar to contact24. When this occurs, there will be no circuit for again energizing thestepping relay E, and the rotary movement of the selector will cease.Also, due to absenceof ground on the test contact, the switching relay Dis nolonger short circuited, and it energizes over a circuit whichincludes the grounded release trunk conductor 3, the winding of relay D,the 01T .normal springs 19, and the vwinding of relay E. The switchingrelay D is of relatively high resistance, so that the stepping relay Edoes not energize in series with it. Upon energizing, relay D makes theselected trunk busy by extending ground to the test wiper and testcontact at armature 16. At armatures 15 and 18 relay' D disconnects theincoming line conductors from the line relay A and continues them.through to the wipers 20 and 22, whence the line circuit extends overcontacts 23 and 25 and trunk conductors 26 and 28 to the line relays ofthe next selector. In the next selector, it will be understood, the linerelay now energizes, followed by the energization of the release relay,which places ground on conductor 27 in order to maintain the relay Denergized and in order to maintain ground on the release trunk conductor3 incoming to the selector S. It is noted that when relay D energizesthe line relay A and the release relay B both deenergize, the latterdisconnecting ground at armature 6 from the release trunk conductor 3.No circuit is closed for the release magnet 8 because ground has beendisconnected at armature 17 of relay D.

'I'he foregoing describes the ordinary operation of the selectorV S incompleting a connection in an automatic telephone system. The operationof the other switches involved need not be especially considered. It isunderstood, of course, that when the connection is to be broken down,the hanging up of the receiver by the calling subscriber results in theremoval of ground from the release trunk conductor 27. This brings aboutthe deenergization of relay D, which closes a circuit for the relaymagnet 8 at armature 17. A double seizure is apt to occur when twoselector switches having access to the same group of trunks start tohunt for an idle trunk simultaneously or nearly simultaneously. It maybe stated that if the two selectors start to hunt exactly together it isalmost certain that they will seize the same trunk, provided the flrsttrunk is idle, and unless there is considerable difference in theadjustment of the relays. f A double seizure also may occur even thoughthe selectors do not Start to hunt exactly together, and the danger is`not entirely eliminated until the time interval by which the hunting ofone selector is delayed after the hunting of the other one reaches acertain value. The value depends upon the adjustment of the relays andother factors and in any case is exceedingly small. In order to explainhow adouble seizure occurs, it may be assumed that the selectors S andS' start to hunt for idle trunks simultaneously. The lnitiationrof thehunting action is caused by the deenergization ofthe relays C and C. Therotary magnets 10 and l0 will therefore be energized simultaneously andit will be assumed that the stepping relays E and E deenergizesimultaneously. Ii the rotary magnets now fall back together, the testcircuits including the relay E and the test wiper 21 of the selector Sand the relay E and test wiper 21' of the selector S will be closedsimultaneously. Neither selector finds ground through its test wiper andaccordingly, the relays D and D will start to pull up simultaneously. Ifthere is not too great difference in adjustment of the relays D and Dthey will pull up substantially together, and double seizure has takenplace.

It will be observed that there are a good many contingencies which mayprevent the double connection. If the switches arrive in testingposition slightly out of synchronism or if the D relays or the E relaysor the rotary magnets are adjusted a little bit differently the doubleconnection may not take place. It should be noted also that there is apossibility for two selectors to start 4to hunt nearly together andinteract and rotate past several idle trunklines until they get out 0fstep with each other and seize adjacent idle trunk lines. This is due tothe fact that each selector arrives at a trunk line with ground on itstest wiper. Assume, for instance, that the selector S' has been operatedto select the trunk line shown and that the switch has just arrived intesting position, the circuit o'f the relay E being closed through theAinterrupter contacts of the rotary magnet to the test wiper 21'. Assumenow that at this instant the selector S is advanced by its rotary magnetto the contacts of the same trunk line. At the moment that the switch Sreaches the trunk line, the stepping relay E has not yet deenergized andground will be placed on the test contact 24 and on its multiple contactin the bank of the selector S. Although the relay D has begun toenergize in the selector S it is now shunted down and the relay E' pullsup instead, thus rotating the selector S off the contact. Theenergization of relay E also grounds the test contact of the rst trunkat the beginning of the second rotary step of Aselector S', thisoperation energizing the stepping relay E of the selector S, which bythis time has deenergized. It will be seen that the selectors mayinteract in this way and pass a number of idle trunk lines provided therelay adjustments are substantially the same on the two selectors. Theoperation does not take place very often however with standardselectors.

The eect of adding the resistance according to this invention will nowbe discussed and the operation thereof will be explained sov far as ispossible. The'resistance should be non-inductive and of a value whichdepends on the value of the resistance of the E and D relays.Considering the selector S, for example, the value of the resistance 30should be such that relay E will not pull up in series with it. This isbecause during the operation of the selector S the test wiper 21 isgrounded through the resistance 30. If the resistance 30 is suicientlylow to allow the relay E to pull up, all trunks would obviously testbusy. It is desirable, however, that the resistance 30 be suiliclentlylow so that with two of these re'- sistances in parallel a relay such asrelay E will operate. It may be stated further that the value of theresistance 30 should be such that the relay E will notoperate in serieswith the resistance 30 and the relay D in parallel. The condition ofhaving a relay such as E pull up in series with two resistances such as30 in` parallel occurs when two selectors such as S and S start to testa trunk line simultaneously or nearly so. Under these conditions groundis supplied to the test conductor of the trunk line through resistances30 and 30 in parallel. Now, if the interrupter contacts of therotarymagnets do not close exactly simultaneously, only one such relayas E will have its circuit closed for an instant, andV this relaytherefore can pull up through the resistances 30 and 30. The conditionof a relay such as E attempting to pull up through the resistance 30 andrelay D in parallel occurs in the normal operation of the switch whenonly one selector tests for an idle trunk. If the trunk is idle theselector should stop and seize it, and the relay E therefore should notenergize, notwithstanding the fact that it is being supplied withcurrent through the associated resistance 30 and the relay D inparallel.

Some variation in the value of resistance 30 is possible even though theabove conditions must be met. A selector which is in common use has a Drelay with a resistance of 1300 ohms, and an E relay with a resistanceof 210 ohms. The E relay is shunted by a non-inductive resistance of1300 ohms. With these resistance valuesof the D and E relays, it isfound that if the resistance 30 has a value of 900 ohms verysatisfactory operation takes'place. The limits vary with the relayadjustments. Within the range of ordinary adjustments the lower limitlis about 500 or 600 ohms, while the upper limit is about 1000 ohms.

'I'he exact way in which the addition of the resistance 30 to theselector circuit prevents double seizure is somewhat obscure. 'I'herelay operations take place so fast and there are so many circuits whichhave to be taken into consideration, and there are so many relayadjustments which alter the conditions that it isdiflicult to ascribethe improved result to any single factor. It may be that the improvedoperation is due to a number of different factors, all, however,involving the addition of these resistances. A brief summary of theresults shown by repeated tests with and without the resistance willperhaps be informatory.

In testing this circuit arrangement two selectors such as'S and S' maybe given access to the same group of trunks, as illustrated in thedrawing, and a special arrangement may be added for breaking the circuitof the C relays simultaneously in order to start the switches in huntingoperation, the switches having been previously raised by hand to thesame level. Tests have been made in this way both with and without theresistance 30 and these tests have been made under the condition ofreleasing both C relays simultaneously and under varying conditions inwhich the release of one C relay is permitted to lag a varying amount oftime behind the other C relay. These tests show that with the resistance30 included it is Very difficult to obtain a double seizure.

Without the resistance 30, when the C relays were released exactlytogether, both selectors invariably selected the first trunk, that is, adouble seizure invariably occurred. Double seizure also occurredfrequently when one C relay was permitted to lag behind the other one byas much as 15 milli-seconds, and double seizures were not entirelyeliminated until the a'mount of lag was increased to about 22milli-seconds. It may be stated that when one C relay is permitted tolag slightly behind, the switches-occasionally interact, and thisappears to beco'me more frequent until the lag is increased suflicientlyso that double seizure is entirely absent.

curred only infrequently even under the condi'-,

tion of releasing the C relays exactly together. It

-would appear that this double connection should occur even with theimproved circuit, if the vC relays are released exactly at the sametime. It is probable, however, that there, was some slight difference inthe adjustment of the relays and the rotary magnets of the selectors. Byintroducing only a slight delay in the release of one of the C relays sothat this relay lagged behind the other one, double connections werepractically all eliminated. A time lag of '7l/2 milll-seconds wassuicient to produce this result. The interaction between the switches ismuch more marked and frequently results in stepping both the switchespast all of the idle trunks. This interaction was observed until the lagwas increased to as much as 36 milli-seconds, which appears to be aboutthe point at which normal operation entirely replaces interaction.

As intimated hereinbefore, it is hard to tell just why the circuit worksthe way it does. It

appears to be certain, however, that the addition y yincreased amount ofinteraction which is observed.

A fair estimate of the results secured by the improved circuit is thatapproximately 85% of the double seizures which would normally occur areprevented by the improved circuit arrangement. As noted, a considerablenumber of double seizures are prevented by an increased frequencyA ofinteraction between two selectors hunting simultaneously, and by anincrease in the extent of such interaction, so that both selectors arefrequently rotated past all of the trunk lines. Of course, this meansthat some calls are lost, but it is much better to have the calls lostin this way than to have them lost because of the double seizure of atrunk. When the selectors rotate past all the trunk lines they arrive inthe eleventh position, in which position the rotation ceases and thesubscribers are given a busy signal. Such calls, therefore, do notbecomea source of complaint. l

The invention having been described, that which is considered to be newand for which the protection of Letters Patent is desired will bepointed out more specifically in the appended claims:

1. In-a. selector switch, an incoming test conductor, a test wiper, aswitching relay connected between said conductor and wiper, and anoninductive resistance normally connected between said conductor andwiper in shunt of said relay.

2. In a selector switch, an incoming test coriductor, a test wiper, aswitching relay normally connected between said conductor and wiper,

contacts on said relay for shifting the connection of the wiper from therelay to said conductor, and a resistance connected in shunt of saidrelay until said contacts are operated.

3. in a selector switch, a test wiper, a stepping relay operated oversaid wiper when the same nds a grounded contact, and means in saidselector for applying ground to said wiper through a resistance whilethe selector is operating, said resistance being high enough so that theground placed on the test wiper thereby will not operate said relay,being low enough, however, so that said relay will operate if connectedto ground through said resistance in parallel with a similar resistancein another selector.

4. In a selector switch, a switching relay having a resistance of about1300 ohms, a stepping relay having a resistance of about 210 ohms, and aresistance which is more than 500 ohms and less than 1000 ohms connectedbetween the incoming test conductor and the test Wiper.

5. In a selector switch, a stepping relay which is operated over thetest wiper of the switch when the wiper engages grounded contacts, aswitching relay which is shunted by the engagement of grounded contactsby said wiper, said switching relay operating in series with saidstepping relay when an ungrounded contact is l engaged, and a resistanceconnected between nosotros said test wiper and ground during the huntingoperation of the selector, the value of the said resistance being suchthat the stepping relay will not operate when supplied with currentthrough said resistance and the switching relay in parallel but willoperate when supplied with current through said resistance in' parallelwith a similar resistance in another switch.

6. In combination, a plurality of selector switches, trunk linesaccessible to said switches, means for causing said selectors to huntfor idle trunk lines, and means in each selector whereby two selectorswhich started to hunt simultaneously are caused to interact and rotateover said trunk lines-whether idle or not.

7. In combination, a plurality of selector switches, trunk linesaccessible to said switches, means for causing said selectors to huntfor idle trunk lines, and means in each selector whereby two selectorswhich start` to hunt simultaneously are prevented from seizing the sameidle trunk line, said means operating by rendering such selectorscapable of readily interacting with each other so that they will rotatepast idle or busy trunks until all the trunks are passed over or one ofthem seizes an idle trunk to the exclusion of the other.

REUBEN RUMINLELT.

